Optical activity of the boson peak and two-level systems in silica-germania glasses

Adding GeO2 to optical fiber quality silica glass produces additional mass, dipole moment, and polarizability variability. There is a factor 2.5 increase in Raman scattering for both the boson peak intensity and the phonon assisted two-level system (TLS) scattering in the most concentrated sample. No additional TLS absorption is observed in the far-infrared region. Two experimental findings stand out for this isomorphous substitution into the SiO2 network: (i) in Raman scattering an acousticlike density of states shift is observed for the boson peak and (ii) apparently, only network modifying impurities can dramatically change the low-frequency TLS optical activity.